In the direct injection diesel engine, recently, there is a trend to employ a so-called open type reentrant cavity to reduce soot problematical in the high speed, high load region. The open type reentrant cavity has a cavity entrance having a relatively large diameter to improve the air utilization in the squish area.
In the open type reentrant cavity, however, the utilization of air tends to become rather worse in the central portion of the cavity, so that it is not always possible to reduce the soot sufficiently.
The reentrant cavity has an annular space for maintaining swirl. The annular space of the reentrant cavity is defined by a combination of a central raised portion shaped like a circular cone or a frustum of a circular cone, and a peripheral lip portion formed in an entrance portion of the cavity. In the reentrant cavity of earlier technology, the inclination of the conical surface of the central raised portion is relatively gentle as disclosed in JP 2013-527360A. However, the conical surface of such a gentle inclination is not contributive to improvement of the utilization of air in the central region of the cavity.